Charged-particle-beam exposure device and charged-particle-beam exposure method

ABSTRACT

An electron gun for emitting an electron beam traveling along a beam axis includes a cathode having a tip, the tip having substantially a circular conic shape and a tip surface substantially at the beam axis, the cathode being applied with a first voltage, an anode having a first aperture substantially on the beam axis and being applied with a second voltage higher than the first voltage, a control electrode having a second aperture substantially on the beam axis and being applied with a voltage lower than the first voltage to control a current of the cathode, the second aperture being larger than the tip surface, a guide electrode having a third aperture substantially on the beam axis, being arranged between the cathode and the anode, and being applied with a voltage higher than the first voltage and lower than the second voltage, the third aperture being smaller than the tip surface, and a lens electrode having a fourth aperture substantially on the beam axis, being arranged between the guide electrode and the anode, and being applied with a voltage lower than the first voltage to form a cross-over image of the electron beam, the fourth aperture being larger than the third aperture.

This application is a division of application Ser. No. 08/680,960, filedJul. 16, 1996, now allowed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a device and a method forexposing an object to a charged-particle beam, and particularly relatesto an improvement in an optical system configuration for thecharged-particle beam.

2. Description of the Prior Art

FIG. 1 is an illustrative drawing showing a schematic configuration of acharged-particle-beam exposure device of the prior art.

The charged-particle-beam exposure device of FIG. 1 includes an electrongun 10A, electromagnetic lenses 21 through 25, a diaphragm 12 having arectangular aperture 12a, a mask 13 having an aperture 13a, a diaphragm14 having a round aperture 14a, a main deflector 32, a sub-deflector 33,and deflectors 26 and 34.

Between the electron gun 10A and a wafer 11 to be exposed to acharged-particle beam, the diaphragm 12, the mask 13, and the diaphragm14 are arranged along the optical axis AX of the charged-particle beam.The electromagnetic lenses 21 through 25 are arranged such that thecharged-particle beam emitted from the electron gun 10A has across-sectional extension as shown by a reference numeral 15. Here, thecross-sectional extension 15 of the charged-particle beam is shownenlarged in a direction perpendicular to the optical axis AX in order toclearly show the path of the charged-particle beam.

The electromagnetic lens 21 includes an electromagnetic lens 21A and anelectromagnetic lens 21B arranged along both sides of the diaphragm 12.The charged-particle beam made parallel with the optical axis AX by theelectromagnetic lens 21A passes through the rectangular aperture 12a ofthe diaphragm 12 so that the cross section of the charged-particle beamis rectangularly shaped. The electromagnetic lens 21B converges thecharged-particle beam having the rectangular cross section.

The electromagnetic lens 22 includes an electromagnetic lens 22A and anelectromagnetic lens 22B arranged along both sides of the mask 13. Thecharged-particle beam made parallel with the optical axis AX by theelectromagnetic lens 22A passes through the aperture 13a of the mask 13.By the aperture 13a, the cross section of the charged-particle beam isshaped into a pattern to be formed on the wafer 11. The electromagneticlens 22B converges the charged-particle beam passing through theaperture 13a.

The electromagnetic lens 23 forms a cross-over image of thecharged-particle beam at the round aperture 14a of the diaphragm 14. Theround aperture 14a of the diaphragm 14 is used for restricting an angleof the charged-particle beam incident onto the wafer 11. Also, thediaphragm 14 is used for blocking the charged-particle beam during anon-exposure period of an exposure process. The electromagnetic lens 23and the electromagnetic lens 24 provide an image-size reduction. Theelectromagnetic lens 25 is an objective lens to form an image on thewafer 11.

The main deflector 32 and the sub-deflector 33 are used for scanning thecharged-particle beam on the wafer 11. The deflector 34 deflects thecharged-particle beam away from the round aperture 14a of the diaphragm14 during the non-exposure period of the exposure process to block thecharged-particle beam.

In a variable-rectangle exposure method, the charged-particle beam isdeflected by the deflector 26 to be displaced on the mask 13. FIG. 2 isan illustrative drawing showing a displaced charged-particle beam on themask 13. As shown in FIG. 2, a displaced cross section 15a of thecharged-particle beam has a portion 16 thereof passing through theaperture 13a of the mask 13. The portion 16 is shown as a hatched regionin the figure. The portion 16 passing through the aperture 13a isreduced in size to be projected onto the wafer 11. In this manner, thecross section of the charged-particle beam is shaped into a rectanglewhose shape and size vary according to a voltage applied to thedeflector 26.

In a block exposure method, a plurality of blocks each having arespective aperture pattern are provided on the mask 13. One of theblocks is selected, and the charged-particle beam is directed to theselected block by the deflector 26. The charged-particle beam passesthrough the aperture pattern of the selected block to have a crosssection accordingly shaped. Then, the shaped cross-section pattern ofthe charged-particle beam is reduced in size to be projected onto thewafer 11. In this manner, one shot of the charged-particle beam cancreate a various fine pattern on the wafer 11.

In the block exposure method, an area exposed to one shot of thecharged-particle beam on the mask 13 is a rectangular block includingone or more apertures, and these apertures together form the aperturepattern of the block. The cross section of the charged-particle beam isshaped in advance to correspond to this rectangular block by therectangular aperture 12a of the diaphragm 12.

FIG. 3 is an illustrative drawing showing a configuration of theelectron gun 10A with a driving circuit. The electron gun 10A of FIG. 3includes a cathode 40A, an anode 41, a Wehnelt 42A, heaters 43 and 44,leads 45 and 46, and an insulator 471. The cathode 40A has a sharplyformed tip to generate a high-voltage electric field. A radius of thecurvature of the tip is about 100 μm. An aperture A0 of the Wehnelt 42Ahas a diameter L4 of about 1.5 mm. A distance L5 between the anode 41and the Wehnelt 42A is about 15 mm.

In order to effect a thermionic emission from the cathode 40A, adirect-current power source 50 is connected between the lead 45 and thelead 46 to heat the cathode 40A. An electron beam EBO emitted from thecathode 40A needs to be accelerated up to a predetermined energy level.In order to effect this acceleration, a plus node of a direct-currentpower source 51 is connected to the anode 41 having an aperture A1 andto the ground, and a minus node of the direct-current power source 51 isconnected to the lead 45 via an ammeter 52, a bias resistor 53, and abias resistor 54, and connected to the lead 46 via the ammeter 52, thebias resistor 53, and a bias resistor 55.

A voltage level of the Wehnelt 42A is controlled by a control circuit 56such that a cathode current I1 detected by the ammeter 52 becomes apredetermined amount. The voltage level of the Wehnelt 42A is lower thanthat of the cathode 40A, and the cathode current I1 supplied to thecathode 40A is controlled along with the voltage level of the Wehnelt42A. A lens effect of the Wehnelt 42A makes the electron beam EBO form across-over image CO.

The electron gun 10A described above has the following problems.

(1) Non-Uniform Distribution of Current Density

A current density of the cross section of the charged-particle beamemitted from the electron gun 10A has substantially a Gaussiandistribution as shown at the top left of FIG. 1. Thus, the diaphragm 12of FIG. 1 cuts off slopes of the Gaussian distribution, and thecharged-particle beam passing through the rectangular aperture 12a ofthe diaphragm 12 has a non-uniform distribution. This non-uniformity ofthe current density distribution creates the following problems.

(a) Decrease in Accuracy of Resist Pattern

Assume that a pattern to be formed in a 5-μm-square block for one shotof the charged-particle beam has a width of 0.1 μm on the wafer 11.Because of the non-uniformity of the current density distribution, thereis an error of about 0.03 μm between a pattern width at a center portionof the block and a pattern width at a perimeter portion of the block.This error restricts the fineness in which the pattern can be made.

Reducing the size of the block may reduce the amount of an error. When aside length of the rectangular aperture 12a of the diaphragm 12 as wellas a side length of the block on the mask 13 is shortened by a ratio of1/1.4, for example, the number of shots of the charged-particle beamrequired for forming a given pattern increases twofold. Thus, reducingthe size of the block results in a significant reduction of thethroughput.

(b) Damage to Diaphragm 12

Because the current-density distribution of the charged-particle beamhas a Gaussian profile, most of the electron beam EBO emitted from theelectron gun 10A is blocked by the diaphragm 12 to be wasted. When acurrent amount input to the diaphragm 12 is 700 μA, for example, thecurrent amount passing through the rectangular aperture 12a of thediaphragm 12 is 20 μA.

Since the electron beam EBO is accelerated by a high voltage such as 50KV applied between the cathode 40A and the anode 41, the diaphragm 12generates a great amount of heat. Also, because of electrons hitting thediaphragm 12, contamination is attached to an edge of the rectangularaperture 12a of the diaphragm 12 to deform an exposure pattern.

Assume that the rectangular aperture 12a is a 150-μm square, a block onthe mask 13 is a 300-μm square, and a pattern of this block gives areduced projection of a 5-μm square on the wafer 11. If a contaminationof a size of 1 μm is attached to an edge of the rectangular aperture12a, an exposure pattern on the wafer 11 has an error of 0.03 μm(=1×5/150). If the pattern has a width of 0.1 μm, the error amounts to30% of the width. A passage of time will accumulate the contamination.Thus, the life of the diaphragm 12 is shortened by the contamination.

(c) Damage to Mask 13

The diaphragm 12 is as thin as 20 μm around an aperture pattern in orderto form a fine pattern, and is not provided in an effective heatreleasing environment. The electron beam passing through the rectangularaperture 12a and yet having a high energy is directed to the mask 13.Since the current density distribution of the electron beam isnon-uniform, the current amount of the electron beam is generallydetermined based on a current density near the perimeter of the beamcross section, where the current density is relatively low. This leadsto an exposure of the mask 13 to an excessive amount of the electronbeam. With this excessive current amount, the mask 13 is excessivelyheated by hitting electrons. The more the degree of non-uniformity, thesmaller the usable maximum amount of the electron beam.

(d) Contamination due to Diaphragm 14

In general, a cross section of an electron beam tends to be expanded bythe Coulomb interaction among the electrons. Because of thenon-uniformity of the current density distribution, a center portion ofthe electron beam tends to have a current density which is excessivelygreat. Therefore, an expansion of the beam cross section is relativelylarge when the beam has the non-uniformity.

The greater the expansion of the beam cross section, the less a currentcan pass through the round aperture 14a of the diaphragm 14. When acurrent amount passing through the rectangular aperture 12a of thediaphragm 12 is 20 μA, for example, the current amount passing throughthe round aperture 14a of the diaphragm 14 is about 10 μA. Electronswhich cannot pass through the round aperture 14a cause contamination tobe attached to an inside surface of the electron-beam-exposure column.Also, these electrons cause electrical charge of theelectron-beam-exposure column, and this electrical charge causes adisturbance of the electrical field to vent the electron beam.

As previously described, the electron beam is deflected by the deflector34 to be blocked by the diaphragm 14 during the non-exposure period ofthe exposure process. Since the beam cross section is expanded asdescribed above, the deflection amount of the deflector 34 should beincreased in order to effectively block the entire electron beam. Thismakes it difficult to achieve a high-speed blanking operation, and leadsto a reduction of the throughput.

(2) Shortened Life of Electron Gun

Some of the positive ions generated from the diaphragm 12 or the anode41 by the hitting of electrons are driven by the high-voltage electricalfield between the anode 41 and the cathode 40A. These positive ions hithard the tip of the cathode 40A to deform the shape of the tip. Thisresults in a deterioration of emission characteristics of the electronbeam emitted from the cathode 40A. That is, the life of the electron gun10A is shortened.

(3) Difficulties in Adjustment of Electron Gun

Since the cathode current I1 is controlled to be constant by the controlcircuit 56, the voltage level of the Wehnelt 42A is varied according toa change in the tip shape and temperature of the cathode 40A. Along witha variation in the voltage level, a position of the cross-over image Coand an electron-beam-emission boundary B at the tip of the cathode 40Aare changed. This leads to changes in the current density distributionon the mask 13 and in the current amount of the electron beam passingthrough the diaphragm 14. In order to achieve an appropriate exposure,the settings of the voltage level of the direct-current power source 51and the cathode current I1 need to be changed accordingly. Parametersinvolved in the control of these settings, however, are intertwined witheach other, so that the adjustment is extremely difficult.

These problems described above are present not only in thevariable-rectangle method or in the block-exposure method but also in ablanking-aperture-array method, which uses in the place of the mask 13 ablanking-aperture array having a number of arranged apertures togenerate a set of micro electron beams forming a desired pattern on thewafer 11.

Accordingly, there is a need for an electron gun which is easy to beadjusted, has a long life, and emits an electron beam having arelatively uniform current density distribution, and, also, a need for acharged-particle-beam exposure device and a charged-particle-beamexposure method which use this electron gun.

In the following, the problem of the damage to the mask 13 will bedescribed in further detail.

FIG. 4A is a cross-sectional view of the mask 13 taken along a planeincluding the optical axis AX. The aperture 13a is formed through a thinplate of silicon by a photolithography technique. Edges of the aperture13a can be sharpened to a fineness of an atom-size level, so that adepth of the aperture 13a in a direction of the optical axis AX can bevirtually zero. When an energy of the electron beam is relatively low, aprojected image of the aperture 13a on the wafer 11 is very sharp. Asfor dimensions, the aperture 13a is a 200-μm square, and a reductionratio is 1/100, for example. When the portion 16 of the beam crosssection (FIG. 2) is a 10-μm square, the projected image on the wafer 11will be a 0.1-μm square.

The lower the energy of the electron beam, the greater the exposureamount by electrons forwardly scattered in a resist layer on the wafer11 and by electrons backwardly scattered in a silicon substrate tore-enter the resist layer. Thus, the lower the energy of the electronbeam, the larger the exposure area and the less concentration of theexposure-intensity distribution on the wafer 11.

In order to sharply form a fine pattern such as having a width of 0.1μm, a high-energy electron beam such as that having an energy of 50 KVshould be used. In this case, the electron beam partially penetratesthrough thin edges of the aperture 13a, so that a current densitydistribution on the wafer 11 has tapering-off slopes. FIG. 4B is anillustrative drawing showing the current density distribution on thewafer 11. The tapering off of the current density distribution on thewafer 11 leads to a reduced sharpness of the exposure pattern.

Also, there is a possibility that the mask 13 is melted by the electronbeam. When this happens, the aperture 13a cannot be used any more. Areduction in the current amount of the electron beam can prevent themelting of the mask 13, but leads to a longer exposure time to assure arequired exposure amount, thereby decreasing the throughput. The higherthe energy of the electron beam, the smaller the exposure area on thewafer 11. Thus, a higher energy of the electron beam leads to anincreased reduction in the throughput.

FIG. 4C is a cross-sectional view of an aperture formed by coating a Talayer on the mask 13. An aperture 13b of FIG. 4C includes the mask 13and a Ta layer 131 coated on the mask 13. Ta is a heavy metal having ahigh melting point, having a feature of keeping a sturdy contact withSi, and having a thermal expansion coefficient close to that of Si.Electrons of 50 KV travel 16.9 μm on average in Si before they arestopped by collisions. Whereas electrons of 50 KV travel only 2.4 μm onaverage in Ta. The melting point of Si is 1410° C., while that of Ta isas high as 2990° C.

When the electron beam of 50 KV is used, the electrons hit the Ta layer131 so severely, and a slight difference in thermal expansioncoefficients between the Ta layer 131 and the silicon becomessignificant. As a result, the Ta layer 131 is broken off from the mask13, so that the edges of the aperture 13a are melted by the electronbeam.

Mo is known as a heavy metal having a high melting point and easilyprocessed. When an aperture is formed through Mo, however, curvatureshaving a radius ranging from 10 μm to 20 μm are created at the cornersof the aperture. These curvatures are out of a tolerance range of 0.5μm.

There is a method of obviating this problem, which is disclosed inJapanese Laid-open Patent Application No. 59-111326. FIGS. 5A through 5Care illustrative drawings showing the method of creating a rectangularaperture without the aperture rounding curvatures. As shown in FIGS. 5Aand 5B, a slit 132 and a slit 133 are formed through a mask 13C1 and amask 13C2, respectively. Then, as shown in FIG. 5C, the mask 13C1 andthe mask 13C2 are overlaid one over the other to form a mask 13C suchthat the slits 132 and 133 form a cross. The aperture 13a of the mask13C is a sharp rectangle without curvatures at the corners.

The slits 132 and 133 should have a width in a range between 200 μm and500 μm, and, unfortunately, such slits cannot be formed by a mechanicalprocess. Instead of a mechanical process, a dry etching process needs tobe used. Thus, the thickness of the masks 13C1 and 13C2 needs to be asthin as an order of ten μm. With this thickness, the mask 13C are toofragile, and it is difficult to mount the mask 13C to a holder withoutdistorting the mask 13C.

Japanese Laid-open Patent Application No.59-111326 also discloses aformation of a rectangular aperture by overlaying four Mo discs one overanother. In this formation, edges of the rectangular aperture are toothick to form a sharp exposure image on the wafer.

Accordingly, there is a need for a charged-particle-beam exposure deviceand a charged-particle-beam exposure method which can use ahigh-energy-charged-particle beam to achieve a sufficient sharpness ofan exposure pattern, and a need for a sturdy mask easy to bemanufactured and a method of manufacturing the sturdy mask.

In the following, some of the above-identified problems will bedescribed further in detail.

FIG. 6 is an illustrative drawing showing an exposure-column unit 110 ofa charged-particle-beam exposure device of the block exposure type ofthe prior art.

In FIG. 6, the exposure-column unit 110 includes a charged-particle-beamgenerator 114 having a cathode 111, a grid (Wehnelt) 112, and an anode113. The exposure-column unit 110 further includes a first slit 115providing a rectangular shape to the charged particle beam, and a firstlens 116 converging the shaped beam. The exposure-column unit 110further includes second and third lenses 118 and 119 opposing eachother, a mask 120 mounted movably in a horizontal direction between thesecond and third lenses 118 and 119.

On the mask 120 are provided a plurality of blocks having variousaperture patterns. One of the blocks are selected, and first-to-fourthdeflectors 121 through 124 deflect the beam to the selected block. Thecharged-particle beam passing through an aperture pattern of theselected block has a cross section shaped into the aperture pattern.

The exposure-column unit 110 further includes a blanking 125 blanking orpassing the beam according to a blanking signal, a fourth lens 126converging the beam, a round aperture 127, and a fifth lens 129. Theexposure-column unit 110 further includes an objective lens 132projecting the beam onto a wafer W, and a main deflector 133 and asub-deflector 134 positioning the beam on the wafer W. Theexposure-column unit 110 further includes a stage 135 carrying the waferW to move it in horizontal directions.

The configuration of FIG. 6 can be used for the variable-rectanglemethod and the blanking-aperture array method by replacing the mask 120with a respective mask.

In the charged-particle-beam exposure device of the prior art, elementsrestricting the current amount of the electron beam emitted from theelectron-beam generator 114 include the first slit 115, the mask 120,and the round aperture 127.

The electron beam having a current amount of several hundreds of μA whenemitted from the electron-beam generator 114 is partially cut off by thefirst slit 115, such that the electron beam passing through the firstslit 115 has a current amount of several tens of μA. This electron beamwith the current amount of several tens of μA is directed to the mask120. When an applied voltage is 50 KV and the current amount is 20 μA,for example, the mask 120 may generate heat of 1.0 W.

Among the three elements restricting the current amount of the electronbeam, the first slit 115 and the round aperture 127 are made of metalsuch as molybdenum or tungsten. It is not likely that they are melted byheat. The mask 120 is made of silicon, however, because of a need toform fine apertures based on the semiconductor technology. Since themelting point of silicon is 1440° C., the mask 120 may be melted throughheat generated by the electron beam exposure.

Thus, as previously described, the charged-particle-beam exposure deviceof the prior art has a problem in that the mask can be melted due to alarge current amount of the electron beam.

The electron beam is also partially cut by the round aperture 127. Theround aperture 127 serves to partially cut off a cross-over image torestrict an angle of the electron beam incident onto the wafer. Here,the cross-over image is an image of the electron-beam generator 114, andthe partial cutting off of the electron beam at a position where thecross-over image is formed does not affect an image of the aperturepattern of the mask 120.

The round aperture 127 is also used for completely cutting (blanking)the electron beam. When the electron beam is to be blanked, the blanking125 deflects the electron beam such that the electron beam is shiftedaway from the aperture of the round aperture 127. Unfortunately, theelectron beam has a Gaussian distribution in the cross section thereofas described earlier. In order to shift the electron beam completely offthe aperture of the round aperture 127, the blanking 125 needs to bringabout a large deflection of the electron beam. Therefore, a high voltageneeds to be applied to the blanking 125, leading to a difficulty inachieving a high-speed blanking operation.

Part of the electron beam cut off by the round aperture 127 is not usedfor exposing the wafer, and, thus, is an excessive portion. However,this excessive portion of the electron beam hinders the high-speedblanking operation.

Therefore, the charged-particle-beam exposure device of the prior arthas a problem in that the excessive portion of the electron beam hindersthe high-speed blanking operation.

Furthermore, an adverse effect of the excessive portion of the electronbeam can be found in accumulation of contamination. The larger thecurrent amount of the electron beam, the more likely the contaminationsuch as dust floating in the exposure-column unit 110 is hit byelectrons to be attached to various elements of the exposure-column unit110. Also, it is more likely that charge is built up at thecontamination attached to the various elements. Such a charge is notdesirable since it distorts a trajectory of the electron beam.

Accordingly, there is a need for a charged-particle-beam exposure devicein which an excessive portion of the electron beam is cut off to preventthe melting of elements, to enable a high-speed blanking operation, andto reduce a possibility of accumulation and charging up ofcontamination.

As can be seen from the previous description, the non-uniform currentdensity distribution and high energy of the electron beam causes variouscompounding problems in the charged-particle-beam exposure device of theprior art.

In summary, there is a need for an electron gun which is easilyadjusted, has a long life, and emits an electron beam having arelatively uniform current density distribution, and, also, a need for acharged-particle-beam exposure device and a charged-particle-beamexposure method which use this electron gun.

Also, there is a need for a charged-particle-beam exposure device and acharged-particle-beam exposure method which can use ahigh-energy-charged-particle beam to achieve a sufficient sharpness ofan exposure pattern, and a need for a sturdy mask easy to bemanufactured and a method of manufacturing such a sturdy mask.

Further, there is a need for a charged-particle-beam exposure device inwhich an excessive portion of the electron beam is cut off to preventthe melting of elements, to enable a high-speed blanking operation, andto reduce a possibility of accumulation and charging up ofcontamination.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to providea charged-particle-beam exposure device and a charged-particle-beamexposure method which can satisfy the needs described above.

It is another and more specific object of the present invention toprovide an electron gun which is easily adjusted, has a long life, andemits an electron beam having a relatively uniform current densitydistribution, and, also, to provide a charged-particle-beam exposuredevice and a charged-particle-beam exposure method which use thiselectron gun.

In order to achieve the above objects according to the presentinvention, an electron gun for emitting an electron beam traveling alonga beam axis includes a cathode having a tip, the tip havingsubstantially a circular conic shape and a tip surface substantially ata beam axis, the cathode being applied with a first voltage, an anodehaving a first aperture substantially on the beam axis and being appliedwith a second voltage higher than the first voltage, a control electrodehaving a second aperture substantially on the beam axis and beingapplied with a voltage lower than the first voltage to control a currentof the cathode, the second aperture being larger than the tip surface, aguide electrode having a third aperture substantially on the beam axis,being arranged between the cathode and the anode, and being applied witha voltage higher than the first voltage and lower than the secondvoltage, the third aperture being smaller than the tip surface, and alens electrode having a fourth aperture substantially on the beam axis,being arranged between the guide electrode and the anode, and beingapplied with a voltage lower than the first voltage to form a cross-overimage of the electron beam, the fourth aperture being larger than thethird aperture. The electron gun described here is also used in acharged-particle-beam exposure device and a charged-particle-beamexposure method for exposing a wafer to a charged-particle beam.

In the electron gun described above, the guide electrode has an apertureat the optical axis smaller than the tip surface of the cathode, so thatthe electron beam emitted from the tip and passing through the aperturehas a relatively uniform distribution of the current density. Since anacceleration of electrons up to the guide electrode is much smaller thanthat for the anode, the heat generation at the guide electrode isrelatively small. Also, even when positive ions colliding with the tipsurface of the cathode deforms the shape of the cathode tip, there islittle influence on the emission characteristics of the electron beam.Thus, the maintenance of the electron gun is easier, and the lifethereof will be elongated. Furthermore, a change in the voltage level ofthe control electrode for the purpose of changing a current of theelectron beam does not bring about changes in the position of thecross-over image and the shape and current density distribution of theelectron beam. Since the position of the cross-over image is determinedby the voltage level of the lens electrode, the adjustment of theelectron gun is easier.

It is yet another object of the present invention to provide acharged-particle-beam exposure device and a charged-particle-beamexposure method which can use a high-energy-charged-particle beam toachieve a sufficient sharpness of an exposure pattern, and to provide asturdy mask which is easily manufactured and a method of manufacturingsuch a sturdy mask.

In order to achieve the above objects according to the presentinvention, a mask having at least one aperture is used in a device forexposing a wafer to a charged-particle beam, which device passes thecharged-particle beam through the at least one aperture to shape a crosssection thereof before exposing the wafer to the charged-particle beam.The mask includes first through fourth plates each having an edgesurface which is a remaining side surface of a corresponding one of thefirst through fourth plates left after beveling one of edges of acorresponding one of the first through fourth plates, and a holder onwhich the first through fourth plates are fixedly mounted, the first andsecond plates being arranged in parallel with each other with the edgesurface of the first plate facing the edge surface of the second plate,the third and fourth plates being arranged in parallel with each otherwith the edge surface of the third plate facing the edge surface of thefourth plate, the first and second plates being arranged perpendicularto the third and fourth plates so that the edge surfaces of the firstthrough fourth plates form a rectangular aperture as the at least oneaperture of the mask.

Also, according to the present invention, a method of forming the abovemask is provided. The method includes the steps of forming first throughfourth plates, beveling an edge of a side surface of each of the firstthrough fourth plates to form an edge surface which is a remainingportion of the side surface left after the beveling, and fixedlymounting the first through fourth plates on a holder, the first andsecond plates being arranged in parallel with each other with the edgesurface of the first plate facing the edge surface of the second plate,the third and fourth plates being arranged in parallel with each otherwith the edge surface of the third plate facing the edge surface of thefourth plate, the first and second plates being arranged perpendicularto the third and fourth plates so that the edge surfaces of the firstthrough fourth plates form a rectangular aperture as the at least oneaperture of the mask.

In the mask described above, the plates for forming the aperture can beformed from a heavy metal having a high melting point and easilymechanically processed. Thus, the mask is easily manufactured, and ahigh-energy electron beam can be used. Also, use of an appropriate edgewidth makes it possible to provide a sufficient sharpness for an exposedpattern. Namely, the edge width can be made greater than a range whichcharged particles travel on average in the material used for the platesbefore they are stopped by collision with the material, so that thecharged-particle beam does not penetrate through edges of the apertureto deteriorate sharpness of an exposed pattern. Yet, the edge width ofthe aperture is within a possible manufacturing range of the mechanicalprocess, so that the edge width does not become excessively thick todull the sharpness of the exposed pattern.

It is still another object of the present invention to provide acharged-particle-beam exposure device in which an excessive portion ofthe electron beam is cut off to prevent the melting of elements, toenable a high-speed blanking operation, and to reduce a possibility ofaccumulation and charging up of contamination.

In order to achieve the above objects according to the presentinvention, a device is provided which exposes a wafer to acharged-particle beam after shaping a cross section of thecharged-particle beam generated from a generator by passing thecharged-particle beam through at least one aperture. The device includesan electromagnetic lens for forming a cross-over image, a first platehaving a round aperture for shaping a cross section of thecharged-particle beam by cutting off a peripheral portion of thecross-over image, and a second plate having a beam-reduction aperturefor reducing a current amount of the charged-particle beam, the secondplate being located further upstream than the first plate with respectto the charged-particle beam, wherein the charged-particle beam isdirected to the at least one aperture after passing through the roundaperture.

In the device described above, the round aperture cuts off theperipheral portion of the cross-over image to shape the cross section ofthe electron beam, and the beam-reduction aperture located furtherupstream reduces the current amount of the electron beam so as toprevent the first plate of the round aperture from melting through heatgeneration. Therefore, when the electron beam passing through the roundshaping aperture is directed to the mask, there is no risk that the maskis melted by heat generation. Also, since an excessive portion of theelectron beam is cut off by the round aperture, the likelihood of theaccumulation and charging up of the contamination is reduced.

Also, the size of the round shaping aperture can be determined such thatwhen the electron beam shaped by the round aperture reaches an aperturefor the blanking operation, the cross-sectional size of the electronbeam is about the same as the size of the aperture. Therefore, when theblanking operation is conducted by deflecting the electron beam to cutoff the electron beam with the round aperture, the high-speed blankingoperation is achieved.

Other objects and further features of the present invention will beapparent from the following detailed description when read inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustrative drawing showing a schematic configuration of acharged-particle-beam exposure device of the prior art;

FIG. 2 is an illustrative drawing showing a displaced charged-particlebeam on a mask of FIG. 1;

FIG. 3 an illustrative drawing showing a configuration of an electrongun of FIG. 1 with a driving circuit thereof;

FIG. 4A is a cross-sectional view of the mask taken along a planeincluding an optical axis;

FIG. 4B is an illustrative drawing showing a current densitydistribution on a wafer of FIG. 1;

FIG. 4C is a cross-sectional view of an aperture formed by coating a Talayer on the mask;

FIGS. 5A through 5C are illustrative drawings showing a method ofcreating a rectangular aperture without aperture rounding curvatures;

FIG. 6 is an illustrative drawing showing an exposure-column unit of acharged-particle-beam exposure device of a block exposure type of theprior art;

FIG. 7 is a cross-sectional view of an electron gun taken along a planeincluding an optical axis of an electron beam along with a drivingcircuit of the electron gun, according to an embodiment of a firstprinciple;

FIG. 8A is an illustrative drawing showing trajectories of electrons ofan electron beam EB1 guided from a cathode to a guide electrode of FIG.7;

FIG. 8B is an illustrative drawing showing a current densitydistribution of the electron beam EB1 on a surface of the guideelectrode;

FIGS. 9A and 9B are illustrative drawings showing a mask according to afirst embodiment of a second principle of the present invention;

FIGS. 10A through 10C are illustrative drawings showing the mask withplates thereof attached to a holder:

FIGS. 11A through 11C are illustrative drawings showing a mask accordingto a second embodiment of the second principle of the present invention;

FIG. 12 is an illustrative drawing showing an optical system of achanged-particle-beam exposure device according to a first embodiment ofthe third principle of the present invention;

FIG. 13 is an illustrative drawing showing an enlarged view of theoptical system of FIG. 12;

FIG. 14 is illustrative drawing showing a shape of an electron beam whena beam-cutting-off aperture of FIG. 13 is a large aperture;

FIG. 15 is an illustrative drawing showing a shape of the electron beamwhen the beam-cutting-off aperture is a small aperture;

FIG. 16 is an illustrative drawing showing a configuration of acharged-particle-beam exposure device according to a second embodimentof the third principle of the present invention; and

FIG. 17 is an illustrative drawing showing a configuration of acharged-particle-beam exposure device according to a third embodiment ofthe third principle of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, principles and embodiments of the present inventionwill be described with reference to the accompanying drawings.

FIG. 7 is a cross-sectional view of an electron gun 10 taken along aplane including an optical axis of the electron beam along with adriving circuit of the electron gun 10, according to an embodiment of afirst principle. In FIG. 7, a scale of each element is not a reflectionof a real dimension.

A cathode 40 is made of LaB₆, and has an upper part formed in a prismshape and a lower part formed substantially in a circular conic.Opposing the tip of the cathode 40, a guide electrode 48 is provided.The guide electrode 48 is rotationally symmetric around the opticalaxis, and has an aperture A3 at an intersection with the optical axis.As for dimensions, the tip of the cathode 40 has a diameter L2 of 300μm, and a diameter L3 of the aperture A3 is 30 μm, for example.

Around the aperture A3, the guide electrode 48 has a flat portion, whichis positioned in parallel with a flat surface of the tip of the cathode40. The diameter of the tip of the cathode 40 is larger than thediameter of the aperture A3. A voltage level V3 of the guide electrode48 is set higher than a voltage level V1 of the cathode 40 to guide anelectron beam EB1 from the cathode 40.

FIG. 8A is an illustrative drawing showing trajectories of electrons ofthe electron beam EB1 guided from the cathode 40 to the guide electrode48, and FIG. 8B is an illustrative drawing showing a current densitydistribution of the electron beam EB1 on the surface of the guideelectrode 48.

As shown in FIG. 8A, the electrons of the electron beam EB1 havetrajectories perpendicular to the flat portion of the guide electrode48. As shown in FIG. 8B, the electron beam EB1 has an almost uniformdistribution of the current density on the guide electrode 48 within anarea corresponding to the diameter L2 of the cathode 40. Thus, anelectron beam EB2 passing through the aperture A3 has a uniform currentdensity distribution.

Accordingly, some of the problems described in the background of theinvention are addressed as follows.

(1) An accuracy of a resist pattern obtained through the exposureprocess is enhanced so that a finer pattern can be created.

(2) A current amount of the electrons hitting the diaphragm 12 issignificantly reduced, so that the accumulation and charging up of thecontamination are reduced. Thus, the diaphragm 12 has an elongated life.

(3) Excessive electrons of the electron beam colliding with the mask 13are reduced, so that a maximum energy cap of the electron beam for notmelting the mask 13 can be set higher. Thus, the mask 13 has a elongatedlife.

(4) There is an increase in the ratio of the current passing through theround aperture 14a of the diaphragm 14 to the current not passingthrough, so that the accumulation and charging up of the contaminationare reduced. Thus, a high-speed blanking operation is achieved by usingthe deflector 34 and the diaphragm 14.

In FIG. 7, the guide electrode 48 is positioned much closer to thecathode 40 than to the anode 41, so that it is sufficient to use thevoltage level V3 of -49 KV when the voltage level V1 is -50 KV, forexample. In this case, the energy of the electron beam EB1 at the guideelectrode 48 is 1/50 of that at the anode 41. With this energy level,heat generated by the electron beam EB1 hitting the guide electrode 48is relatively small.

As shown in FIG. 7, a control electrode 49 is arranged generally flushwith the flat surface of the tip of the cathode 40. The controlelectrode 49 is provided in order to control a current amount of theelectron beam EB1 emitted from a tip surface S1 of the cathode 40 (seeFIG. 8A), to suppress electrons emitted from a side surface S2 of thecathode 40, and to prevent the electrons emitted from the side surfaceS2 to reach the guide electrode 48. The control electrode 49 isrotationally symmetric around the optical axis, and has an aperture A2at a center thereof.

A diameter L1 of the aperture A2 is larger than the diameter L2, and maybe 1 mm, for example. A voltage level V2 lower than the voltage level V1is applied to the control electrode 49, and may be -51 KV, for example.

As shown in FIG. 7, a lens electrode 42 is arranged nearer to the guideelectrode 48 between the guide electrode 48 and the anode 41 to form across-over image CO of the electron beam EB2 passing through theaperture A3. The lens electrode 42 is rotationally symmetric around theoptical axis, and has an aperture A4 at the center thereof, The apertureA4 allows the passage of the electron beam, and may have a diameter L4of 1 mm, for example.

The control electrode 49 and the lens electrode 42 together play a roleof the Wehnelt 42A of FIG. 3. The role of the Wehnelt 42A is nowseparately taken care of by the lens electrode 42 and the controlelectrode 49 in the electron gun 10 of FIG. 7, so that it is easier toadjust the electron beam. In the electron gun 10A of FIG. 3, a change inthe voltage level of the Wehnelt 42A for the purpose of changing anemitted current brings about changes in the current densitydistribution, the position of the cross-over image CO, and the positionof the electron-beam-emission boundary B. On the other hand, in theelectron gun 10 of FIG. 7, even when the voltage level V2 of the controlelectrode 49 is changed to change the current of the electron beam EB2,neither the position of the cross-over image CO nor the shape of theelectron beam passing through the aperture A3 changes. The position ofthe cross-over image CO is dependent on the voltage level V4 of the lenselectrode 42. Therefore, the adjustment of the electron gun 10 isrelatively easy.

Also, in the electron gun 10A of FIG. 3, the emission characteristics ofthe electron beam are sensitive to relative positions of the cathode 40Aand the Wehnelt 42A and to the shape of the tip of the cathode 40A,which is subjected to a deformation caused by colliding positive ions.Thus, the assembly and maintenance of the electron gun 10A is not easy.In the electron gun 10 of FIG. 7, on the other hand, a tolerance rangeof the position of the cathode 40 for acceptable emissioncharacteristics of the electron beam is relatively wide. Thus, theassembly of the electron gun 10 is easier. Also, even if positive ionscolliding with the tip surface of the cathode 40 cause a deformation ofthe tip surface, the emission characteristics of the electron beamsuffer little influence. Thus, the maintenance of the electron gun 10 iseasier, and the life of the electron gun 10 is elongated.

The heaters 43 and 44 are rectangular solids. The leads 45 and 46 aresupported by the insulator 471 having a disc shape. The controlelectrode 49 and the guide electrode 48 are insulated from each other byan insulator 472, and the guide electrode 48 and the lens electrode 42are insulated from each other by an insulator 473. In FIG. 7, thedirect-current power sources 50 and 51, the ammeter 52, the biasresistors 53 through 55, and the control circuit 56 are the same asthose of FIG. 3.

A current detected by the ammeter 52 is a difference (I1-I2) between thecurrent I1 supplied to the cathode 40 and the current I2 supplied to theguide electrode 48. The control circuit 56 controls the voltage level V2of the control electrode 49 such that the difference (I1-I2) becomesconstant. In this manner, the current emitted from the electron gun 10is kept constant without changing the position of the cross-over imageand the shape and current density distribution of the electron beam.

Between an input of the control circuit 56 and the guide electrode 48, adirect-current power source 57 is provided. Also, a direct-current powersource 58 is provided between a minus output of the direct-current powersource 51 and the lens electrode 42. Output voltages of thedirect-current power sources 50, 51, 57, and 58 are independentlyadjustable.

The first principle of the present invention has been described withreference to a particular embodiment. However, the first principle ofthe present invention is not limited to this embodiment, and variousmodifications and variations are intended to be within a scope of thefirst principle of the present invention.

In the above-described embodiment, the difference (I1-I2) is controlledto be constant. An alternate configuration may be such that the currentI1 is detected and the control circuit 56 controls the voltage level V2of the control electrode 49 so as to keep the current I1 constant.Furthermore, the tip surface of the cathode 40 may be generally flatonly in an area corresponding to the aperture A3, and may be a curvedsurface in its entirety.

As described above, according to the first principle of the presentinvention, the guide electrode is provided to oppose the cathode havinga tip surface, and has an aperture at the optical axis smaller than thetip surface of the cathode, so that the electron beam passing throughthe aperture has a relatively uniform distribution of the currentdensity. The guide electrode is used for guiding electrons from thecathode. Since an acceleration of electrons up to the guide electrode ismuch smaller than that for the anode, the heat generation at the guideelectrode is relatively small. A tolerance range of the position of thecathode for acceptable emission characteristics of the electron beam isrelatively wider, so that the assembly of the electron gun is easier.

Also, even when positive ions colliding with the tip surface of thecathode deform the shape of the cathode tip, there is little influenceon the emission characteristics of the electron beam. Thus, themaintenance of the electron gun is easier, and the life thereof will beelongated. Furthermore, a change in the voltage level of the controlelectrode for the purpose of changing a current of the electron beamdoes not bring about changes in the position of the cross-over image andthe shape and current density distribution of the electron beam. Sincethe position of the cross-over image is determined by the voltage levelof the lens electrode, the adjustment of the electron gun is easier.

In the following, a second principle and embodiments thereof will bedescribed with reference to the accompanying drawings. The secondprinciple of the present invention is intended to be used in thevariable-rectangle method.

FIGS. 9A and 9B are illustrative drawings showing a mask 230 accordingto a first embodiment of a second principle of the present invention.FIG. 9A is an isometric view of the mask 230, and FIG. 9B is an enlargedpartial front view of the mask 230.

In FIG. 9A, the mask 230 includes plates 231 through 234 which areidentical to each other. The plates 231 through 234 are formed from aheavy metal having a high melting point and easily mechanicallyprocessed, e.g., Mo with a melting point of 2620° C. The plate 231 is along rectangular plate, and one of the side surfaces thereof extendingin a longitudinal direction is beveled by a mechanical process. Thus, asloping surface 311 and a edge surface 312 are created by the beveling.A lapping process is applied to the edge surface 312 to obtain a smoothsurface with surface irregularity under about 1 μm.

At both ends of the plate 231, elongated holes 313 and 314 are formed toreceive a fixing bolt. A longitudinal direction of the elongated holes313 and 314 is the same as the longitudinal direction of the plate 231.The elongated holes 313 and 314 are used for fixedly mounting the plate231 to a holder by adjusting the position of the plate 231 in thelongitudinal direction such that the surface irregularity of the edgesurface 312 at a point of an aperture 230a becomes smaller than apredetermined limit.

In the same manner, the plate 232 is provided with a sloping surface321, an edge surface 322, and elongated holes 323 and 324. The plate 233is provided with a sloping surface 331, an edge surface 332, andelongated holes 333 and 334. The plate 234 is provided with a slopingsurface 341, an edge surface 342, and elongated holes 343 and 344.

The widest surfaces of the plates 231 and 232 are in surface contactwith the widest surfaces of the plates 233 and 234, as shown in FIG. 9A.The edge surface 312 and the edge surface 322 face each other, and theedge surface 332 and the edge surface 342 face each other. The plates231 and 232 are arranged perpendicular to the plates 233 and 234. Theedge surfaces 312, 322, 332, and 342 together form the aperture 230a.

FIGS. 10A through 10C are illustrative drawings showing the mask 230with the plates 231 through 234 attached to a holder 235. As shown inFIG. 10A, the holder 235 has a ring shape.

Preferably, the holder 235 is formed from the same material as that ofthe plates 231 through 234, so that stress is not generated by a thermalexpansion difference. Part of the perimeter of the holder 235 isvertically cut off to create a reference surface 350. Also, the holder235 has a circular hole 353 at a center thereof. By using the referencesurface 350, the holder 235 is held in a chuck. A groove 351 forreceiving the plates 231 and 232 is formed in the holder 235 by using alathe to extend across the circular hole 353 in a directionperpendicular to the reference surface 350. Also, a groove 352 forreceiving the plates 233 and 234 is formed in the holder 235 in adirection perpendicular to the groove 351. A depth of the groove 351 isthe same as the thickness of the plate 231, and a depth of the groove352 is twice as deep as that of the groove 351. The plates 233 and 234are fitted into the groove 352, and, then, the plates 231 and 232 arefitted into the groove 351 to arrange these plates in a configurationshown in FIG. 9A.

Before fixing the plates 231 through 234 to the holder 235 with bolts, adimension D of the surface irregularity of the edge surfaces 312, 322,332, and 342 is measured by a measuring device. Positions of the plates231 through 234 in respective longitudinal directions thereof aredetermined such that the aperture 230a is formed by surfaces having thedimension D of the surface irregularity smaller than a predeterminedvalue. Then, the plates 231 through 234 are fixed to the holder 235 withbolts. That is, bolts 335 and 336 are inserted into the elongated holes333 and 334, and the plate 233 is positioned such that a side surface ofthe plate 233 extending in the longitudinal direction is in surfacecontact with a side surface of the groove 352. Then, the bolts 335 and336 are screwed into holes (not shown) formed in the groove 352 to fixthe plate 233 to the holder 235. In the same manner, the plate 234, theplate 231, and the plate 232 are fixed to the holder 235 with bolts 345and 346, bolts 315 and 316, and bolts 325 and 326, respectively.

Even when the dimension D of the surface irregularity exceeds thepredetermined limit after repeated use over time, the plates 231 through234 can be readjusted in the respective longitudinal directions. Thus,the aperture 230a can be again formed by surfaces having the surfaceirregularity under the predetermined value. Thus, the mask 230 has anelongated (i.e., extended) life.

Also, the holder 235 is provided with round holes 354 through 357. Boltsare inserted through the round holes 354 through 357 to fixedly mountthe mask 230 to the charged-particle-beam exposure device.

In FIG. 9B, a thickness t of the plate 231 is sufficient if the plate231 is sturdy enough to be processed and mounted. When Mo is used, thethickness t should be greater than 500 μm. An angle of the slopingsurface 311 should be such that the sloping surface 311 does notfunctionally become part of the aperture 230a. Preferably, the angle is45° which is easy to process in the beveling. A width e of the edgesurface 312 in a direction of the optical axis is preferably within arange between 10 μm and 100 μm, as will be described later. For example,the width e may be 15 μm. The width e should be greater than a rangewhich electrons of the electron beam travel on average through thematerial before being stopped by collisions. If the width e is smallerthan this range, the electron beam penetrates through the edges of theaperture 230a to deteriorate sharpness of an exposed pattern. A minimumwidth of the width e varies depending on the material used for theplates 231 through 234. The minimum width of the width e is the largestof the lower limit of the mechanical process (e.g., 10 μm for Mo) andthe range of electrons (e.g., 3.9 μm for Mo with a 50 KV electron beam).Typically, the minimum width of the width e is determined by the former.

The dimension D of the surface irregularity of the edge surface 312 andthe upper limit of the width e of the edge surface 312 are determined asfollows.

In FIG. 1, assume that a convergence angle θ of the electron beamincident onto the wafer 11 is 5 mrad, a projection reduction ratio(reduction ratio of length) M from the aperture 13a to the wafer 11 is100, and a tolerable error α of a pattern width on the wafer 11 is 0.01μm.

In this case, the dimension D of the surface irregularity of the edgesurface 312 corresponds to D/M on the wafer 11. Thus, D/M should besmaller than or equal to α.

    D≦α·M=0.01×100=1 μm!        (1)

This condition is achievable by applying a lapping process when Mo isused. An edge width 2e of the aperture 230a in a direction of theoptical axis corresponds to 2e/M in the direction of the optical axis onthe wafer 11, so that the electron beam spreads by (2e/M)·tan θ in adirection perpendicular to the optical axis. Thus, (2e/M)·tan θ shouldbe smaller than or equal to α.

    e≦αM/(2 tan θ)=0.01×100/(2×0.005)=100 μm(2)

This is greater than the lower limit (10 μm) of the mechanical process,so that there is no problem in creating the aperture 230a satisfyingthis condition.

The holder 235 has a thickness of 5 mm, an outer diameter of 24 mm, aninter diameter of 14 mm, a width of the grooves 351 and 352 of 8.2±0.001 mm, a depth of the groove 351 of 1 mm, and a depth of the groove352 of 2 mm, for example. The plate 231 has a length of 22 mm, a widthof 4 mm, a thickness of 1 mm, a slope angle of the sloping surface 311of 450, and an edge width e of 15 μm, for example.

FIGS. 11A through 11C are illustrative drawings showing a mask 230Aaccording to a second embodiment of the second principle of the presentinvention.

In the configuration of FIGS. 10A through 10C of the first embodiment,the holder 235 is formed from the same material (e.g., Mo) as that ofthe plates 231 through 234. Since there is a difference in temperaturebetween a central portion of the plates 231 through 234 and the holder235, however, a stress caused by thermal expansion is relatively large.Thus, even though the plates 231 through 234 are fixed to the holder 235with the bolts, the plates 231 through 234 may suffer displacements ofposition after repeated use of the mask 230.

In the second embodiment of the second principle of the presentinvention, pins 361 through 368 are used for fixing the plates 231through 234 as shown in FIGS. 11A through 11C. First, the plates 231through 234 are fixed to the holder 235 with the bolts after thepositions thereof are determined. Then, holes are formed through boththe plates 231 through 234 and the holder 235. Then, the pins 361through 368 having a diameter slightly larger than that of the holes areinserted into the holes to prevent the displacements of the plates 231through 234. Preferably, the pins 361 through 368 are formed from thesame material as that of the plates 231 through 234 such as Mo.

The second principle of the present invention has been described withreference to particular embodiments. However, the second principle ofthe present invention is not limited to these embodiments, and variousmodifications and variations are intended to be within a scope of thesecond principle of the present invention.

For example, the mask 230 can be used as the diaphragm 12 of FIG. 1.With a reduction rate K of an image from the rectangular aperture 12a tothe mask 13, the upper limits of the dimension D of the surfaceirregularity and the edge width e are K times as large as those figuresdescribed above. For example, K is 2.5. When the mask 230 is used in theplace of the diaphragm 12 in FIG. 1, the charged-particle-beam exposuredevice may have a configuration using a block mask or ablanking-aperture array.

As described above, according to the second principle of the presentinvention, the plates for forming the aperture are formed from a heavymetal having a high melting point and easily mechanically processed.Thus, the mask is easily manufactured, and a high-energy electron beamcan be used. Also, use of an appropriate edge width e makes. it possibleto provide a sufficient sharpness for an exposed pattern.

Also, according to the second principle of the present invention, thepositions of the plates in the longitudinal directions are adjusted suchthat the aperture is formed by edge surfaces having a sufficientsmoothness. Because a desired condition is satisfied by this adjustment,the yield of the mask is improved. Also, even if the edge surfaces aretimeworn and no longer have a required smoothness, readjustment of thepositions of the plates can regain the required condition, therebyelongating the life of the mask.

Also, according to the second principle of the present invention, theedge width e is greater than the range of the charged particles, so thatthe charged-particle beam does not penetrate through the edges of theaperture to deteriorate the sharpness of the exposed pattern. Also, theedge width 2e of the aperture can be sufficiently short, so that theexposed pattern satisfies a desired sharpness.

In the following, a third principle and embodiments thereof will bedescribed with reference to the accompanying drawings.

FIG. 12 is an illustrative drawing showing an optical system 410 of acharged-particle-beam exposure device according to a first embodiment ofthe third principle of the present invention. In FIG. 12, the sameelements as those of FIG. 6 are referred to by the same numerals, and adescription thereof will be omitted.

The optical system 410 of the third principle is located furtherupstream than the first slit 115, which is located the most upstream inthe prior-art optical system. The optical system 410 at this positionforms a cross-over image and partially cuts off the cross-over image toremove an excessive portion of the electron beam. As shown in FIG. 12,the optical system 410 of the present invention includes abeam-cutting-off aperture 411 formed from metal such as Mo, across-over-image forming lens 412, a round shaping aperture 413 formedfrom metal such as Mo, a first alignment coil 414, and a secondalignment coil 415.

An electron beam emitted from the electron-beam generator 114 ispartially cut off by the beam-cutting-off aperture 411 to reduce thecurrent amount thereof. The cross-over-image forming lens 412 forms across-over image at a position of the round shaping aperture 413. Theround shaping aperture 413 partially cuts off the cross-over image.

FIG. 13 is an illustrative drawing showing an enlarged view of theoptical system 410. Dimensions of elements shown in FIG. 13 are examplesof numbers which may be used in the charged-particle-beam exposuredevice in the field. In FIG. 13, a first cross-over image C1 with a50-μm diameter is formed immediately under the electron gun. Thecross-over-image forming lens 412 forms a second cross-over image C2having a 50-μm diameter at a position of the round shaping aperture 413.The round shaping aperture 413 has a round aperture of a 30-μm diameter,and cuts off a peripheral part of the second cross-over image C2 toshape a cross section of the electron beam. The electron beam having theshaped cross section and a reduced current amount is directed to thefirst slit 115.

The round shaping aperture 413 is located at a position where the secondcross-over image C2 is formed, i.e., at a position where the electronbeam is focused to give the greatest energy concentration. At this focuspoint, even such metal as molybdenum or tungsten is at risk of beingmelted by heat due to the energy of the electron beam. The round shapingaperture 413 needs to be a thin plate such as one having a thickness of100 μm since a small aperture about a size of 30 μm needs to be formed.Thus, it is likely that the round shaping aperture 413 is melted by theheat due to the energy of the electron beam. In order to avoid this, thebeam-cutting-off aperture 411 positioned out of the focus pointpartially cuts off the electron beam emitted from the electron gun.Thus, the current amount of the electron beam is reduced before theelectron beam reaches the round shaping aperture 413.

Passing through the beam-cutting-off aperture 411 and the round shapingaperture 413, the electron beam has a current amount reduced from about500 μA to about 30 μA.

Accordingly, the melting of the mask 120 (for forming a pattern to beexposed) is avoided, and the accumulation and charging up ofcontamination are reduced.

The size of the round shaping aperture 413 is determined based on thesize of the round aperture 127 (see FIG. 12) located further downstreamin the flow of the electron beam. That is, the round shaping aperture413 is formed such that the cross-sectional size of the electron beamcoincides with the diameter of the round aperture 127 when the electronbeam having cross-sectional size and shape determined by the roundshaping aperture 413 reaches the round aperture 127.

Accordingly, the deflection amount which is required for completelycutting off the electron beam by the round aperture 127 during ablanking operation can be sustained at a minimum. Thus, it is possibleto use a lower voltage level for the blanking operation to achieve ahigh-speed blanking operation.

In FIG. 13, the beam-cutting-off aperture 411 is a round aperture with a200-μm diameter. As described above, the beam-cutting-off aperture 411serves to reduce the current amount of the electron beam by partiallycutting of the electron beam emitted from the electron-beam generator114, so that the round shaping aperture 413 located at the focus pointdoes not melt. Also, the beam-cutting-off aperture 411 serves to definean incident angle and an output angle of the electron beam with respectto the round shaping aperture 413.

FIG. 14 is an illustrative drawing showing a shape of the electron beamwhen the beam-cutting-off aperture 411 is a large aperture. FIG. 15 isan illustrative drawing showing a shape of the electron beam when thebeam-cutting-off aperture 411 is a small aperture.

As shown in FIG. 14, when the beam-cutting-off aperture 411 is a largeaperture, an incident angle φ_(in) and an output angle φ_(out) of theelectron beam with respect to the round shaping aperture 413 are small.In this case, the electron beam covers a sufficiently wide area on thefirst slit 115, so that the electron beam passing through the first slit115 is complete. Thus, the mask 120 located further downstream iscovered by a sufficiently uniform distribution of the electron beam.

On the other hand, as shown in FIG. 15, when the beam-cutting-offaperture 411 is a small aperture, an incident angle φ_(in) and an outputangle φ_(out) of the electron beam with respect to the round shapingaperture 413 are large. In this case, the electron beam does not cover asufficiently wide area on the first slit 115, so that the electron beampassing through the first slit 115 is incomplete. Thus, the mask 120located further downstream is not covered by a sufficiently uniformdistribution of the electron beam. Also, there is a risk that an imageof the beam-cutting-off aperture 411 and an image of the first slit 115are mistaken with each other.

Accordingly, the beam-cutting-off aperture 411 must satisfy conditionsthat the electron beam passing through the first slit 115 is completeand the mask 120 located further downstream is covered by a sufficientlyuniform distribution of the electron beam.

With reference back to FIGS. 12 and 13, the first alignment coil 414 andthe second alignment coil 415 are provided to align the electron beam,so that the electron beam passes through the beam-cutting-off aperture411 and the round shaping aperture 413. Namely, the first alignment coil414 is used for adjusting the trajectory of the electron beam to pass itthrough the beam-cutting-off aperture 411. Then, the second alignmentcoil 415 is used for adjusting the trajectory of the electron beam topass it through the round shaping aperture 413. In this manner, thefirst alignment coil 414 and the second alignment coil 415 arrangedabove and below the beam-cutting-off aperture 411, respectively, cancarry out an alignment of the electron beam with respect to thebeam-cutting-off aperture 411 and the round shaping aperture 413.

As described above, according to the first embodiment of the thirdprinciple of the present invention, the electron beam emitted from theelectron-beam generator 114 is partially cut off by the beam-cutting-offaperture 411 to reduce the current amount, and, then, the round shapingaperture 413 located at the position of the cross-over image partiallycuts off the cross-over image to shape the cross section of the electronbeam. Therefore, a part of the current which becomes excessive aftergoing downstream in the flow of the electron beam is cut off, so thatthe mask 120 is prevented from melting through heat. Also, theaccumulation and charging up of the contamination can be reduced.Furthermore, when the size of the round shaping aperture 413 matcheswith the size of the round aperture 127, the deflection amount of theelectron beam required during the blanking operation is sustained at aminimum. Therefore, a high-speed blanking operation is achieved.

Also, in the first embodiment of the third principle of the presentinvention, a reduction in the current amount of the electron beam helpsto reduce the influence of Coulomb interactions, in which the electronbeam is blurred at the focus point due to the mutual repulsion ofelectrons. When there is a strong presence of the Coulomb interactions,electrons having negative charge are repulsed by each other to expandthe electron beam at the focus point. The magnitude of the Coulombinteractions is proportional to a product of the current density of theelectron beam and the cross-sectional area of the electron beam, i.e.,is proportional to the total current amount of the electron beam. Thus,the reduction in the current amount according to the first embodiment ofthe third principle can lessen the influence of the Coulombinteractions.

FIG. 16 is an illustrative drawing showing a configuration of acharged-particle-beam exposure device according to a second embodimentof the third principle of the present invention.

In general, ozone O₃ is injected inside an exposure column of thecharged-particle-beam exposure device. Since it is difficult to producepure ozone alone, various gases in addition to ozone are mixed in theexposure column. When these gases are ionized to be positively charged,ions hit the electron-beam generator 114 at high speed. These collisionsof the ions with the electron-beam generator 114 cause deformation ofthe electron emitting surface of the electron-beam generator 114. When ashape of the electron beam is distorted by this deformation, an exposurepattern cannot be created properly on a wafer. Therefore, it isdesirable to keep the electron-beam generator 114 in ahigh-degree-vacuum condition.

In the second embodiment of the third principle of the presentinvention, the beam-cutting-off aperture 411 and round shaping aperture413 of FIG. 12 are used for enhancing the degree of vacuum around theelectron-beam generator 114.

In FIG. 16, the charged-particle-beam exposure device briefly includes afirst chamber 421, a second chamber 422, and a third chamber 425. Thefirst chamber 421 contains the electron-beam generator 114 and the firstalignment coil 414. The second chamber 422 contains the cross-over-imageforming lens 412 and the second alignment coil 415. The third chamber425 is a space in which the wafer W on the stage 135 is exposed to theelectron beam in the same manner as in the prior-artcharged-particle-beam exposure device.

The charged-particle-beam exposure device of FIG. 16 further includes anion pump 423 for pumping ions out of the first chamber 421, a turbo pump424 for pumping gases out of the second chamber 422, and a turbo pump426 for pumping gases out of the third chamber 425. While air is takenout of the third chamber 425 by the turbo pump 426 to create a vacuumcondition therein, ozone is injected through an ozone-injection opening427.

Space inside the first chamber 421 is connected via the beam-cutting-offaperture 411 with space inside the second chamber 422. Also, the spaceinside the second chamber 422 and space inside the third chamber 425 areconnected via the round shaping aperture 413.

Since the round shaping aperture 413 between the second chamber 422 andthe third chamber 425 is a small aperture having a diameter of 30 μm,the turbo pump 424 can make the degree of vacuum inside the secondchamber 422 greater than that of the third chamber 425. The ion pump 423takes out ions which are harmful to the electron-beam generator 114,and, also, keeps the inside of the first chamber 421 at a higher degreeof vacuum than the inside of the second chamber 422.

As described above, according to the second embodiment of the thirdprinciple of the present invention, the beam-cutting-off aperture 411and the round shaping aperture 413 are used along with the ion pump 423and the turbo pump 424 to keep different pressures inside differentchambers, so that the space around the electron-beam generator 114 iskept under the high-degree-vacuum condition. Therefore, theelectron-beam generator 114 free from the damage caused by ioncollisions generates a distortion-free electron beam.

FIG. 17 is an illustrative drawing showing a configuration of acharged-particle-beam exposure device according to a third embodiment ofthe third principle of the present invention. In FIG. 17, the sameelements as those of FIG. 16 are referred to by the same numerals, and adescription thereof will be omitted. The charged-particle-beam exposuredevice of FIG. 17 differs from the charged-particle-beam exposure deviceof FIG. 16 of the second embodiment only in that a cooling mechanism 430is provided for the beam-cutting-off aperture 411.

The cooling mechanism 430 is preferably that of water cooling. Use ofthe cooling mechanism 430 prevents a temperature rise of thebeam-cutting-off aperture 411 and neighboring elements thereof. In thismanner, a temperature rise of the exposure column of thecharged-particle-beam exposure device can be alleviated, so that changesin operation characteristics of the device are sustained at a minimumlevel.

As described above, according to the third principle of the presentinvention, the round shaping aperture cuts off a peripheral portion ofthe cross-over image to shape the cross section of the electron beam,and the beam-cutting-off aperture located further upstream reduces thecurrent amount of the electron beam so as to prevent a plate of theround shaping aperture from melting through heat generation. Therefore,when the electron beam passing through the round shaping aperture isdirected to the mask, there is no risk that the mask is melted by heatgeneration. Also, since an excessive portion of the electron beam is cutoff by the round shaping aperture, the likelihood of the accumulationand charging up of the contamination is reduced.

Also, according to the third principle of the present invention, thesize of the round shaping aperture is such that when the electron beamshaped by the round shaping aperture reaches the round aperture for theblanking operation, the cross-sectional size of the electron beam isabout the same as the size of the round aperture. Therefore, when theblanking operation is conducted by deflecting the electron beam to cutoff the electron beam with the round aperture, the high-speed blankingoperation is achieved.

Also, according to the third principle of the present invention, thesize of the beam-cutting-off aperture is such that an aperture of themask is covered by a uniform distribution of the electron beam passingthrough the round shaping aperture. Thus, even through thebeam-cutting-off aperture partially cuts off the electron beam at aposition out of the focus point, there is no adverse effect on theexposure process for the wafer.

Also, according to the third principle of the present invention, theplate of the round shaping aperture and the plate of thebeam-cutting-off aperture are formed from Mo. Thus, these plates are notmelted by heat generation.

Also, according to the third principle of the present invention, thefirst alignment coil (deflector) and the second alignment coil(deflector) are provided for making the electron beam pass through thebeam-cutting-off aperture and the round shaping aperture. Thus, theadjustment of the first and second alignment coils enables an easyalignment of the electron beam with respect to the beam-cutting-offaperture and the round shaping aperture.

Also, according to the third principle of the present invention, thecooling mechanism is provided for cooling the plate of thebeam-cutting-off aperture, so that a temperature rise of the plate andthe surrounding elements thereof are prevented. Therefore, a temperaturerise of the exposure column of the charged-particle-beam exposure devicecan be alleviated, so that changes in operation characteristics of thedevice are sustained at a minimum level.

Also, according to the third principle of the present invention, thesecond chamber defined by the plate of the round shaping aperture andthe plate of the beam-cutting-off aperture is placed between the thirdchamber for exposing a wafer to the electron beam and the first chambercontaining the electron-beam generator, where the first chamber and thefirst chamber are connected with each other via these two apertures.Therefore, the inside of the first chamber containing the electron-beamgenerator can be maintained in a higher degree of vacuum than the insideof the third chamber.

Also, according to the third principle of the present invention, thedegree of vacuum in the second chamber is greater than that in the thirdchamber. Thus, a high degree of vacuum in the first chamber is easilyachieved.

Also, according to the third principle of the present invention, ozoneis injected into the third chamber. Because of the configuration of thefirst through the third chamber described above, the injection of ozoneinto the third chamber does not affect the electron-beam generator inthe first chamber.

Further, the present invention is not limited to these embodiments, butvarious variations and modifications may be made without departing fromthe scope of the present invention.

What is claimed is:
 1. A mask having at least one aperture and used in adevice for exposing a wafer to a charged-particle beam, said devicepassing said charged-particle beam through said at least one aperture toshape a cross section thereof before exposing said wafer to saidcharged-particle beam, said mask comprising:first through fourth plates,each plate having a respective edge surface which is a remaining portionof a corresponding side surface of the plate after beveling the plate; aholder on which said first through fourth plates are fixedly mounted;and said first and second plates being arranged in parallel with eachother with said edge surface of said first plate facing said edgesurface of said second plate, said third and fourth plates beingarranged in parallel with each other with said edge surface of saidthird plate facing said edge surface of said fourth plate and said firstand second plates being arranged perpendicular to said third and fourthplates so that said respective edge surfaces of said first throughfourth plates form a rectangular aperture as said at least one apertureof said mask.
 2. The mask as claimed in claim 1, wherein each of saidedge surfaces has a width e, a tolerable error of a pattern exposed onsaid wafer being denoted by α, a reduction rate of said rectangularaperture projected on said wafer being denoted by M, an angle of saidcharged-particle beam incident on said wafer being denoted by θ, andwherein said width e, said tolerable error α, said reduction rate M, andsaid angle θ are related as:

    e≦α·M/(2 tan θ),

and said width e is larger than a range of widths through which chargedparticles of said charged-particle beam travel in said first throughfourth plates before being stopped by collisions and is larger than alower limit of a mechanical process for forming said edge surface. 3.The mask as claimed in claim 2, wherein a dimension D of surfaceirregularity of each of said edge surfaces is related as:

    D≦α·M.


4. The mask as claimed in claim 3, further comprising bolts, whereinsaid first through fourth plates have respective, elongated holes formedtherethrough, said elongated holes extending in a direction in whichsaid respective edge surfaces extend, said bolts being inserted throughcorresponding said elongated holes and screwed into correspondingthreaded holes in said holder to fix said first through fourth plates tosaid holder so that positions of said first through fourth plates insaid direction are adjustable, thereby to use a selected, sufficientlysmooth portion of each of said edge surfaces for forming saidrectangular aperture.
 5. The mask as claimed in claim 1, furthercomprising pins inserted through respective holes formed through saidfirst through fourth plates and said holder and fixing said firstthrough fourth plates to said holder.
 6. The mask as claimed in claim 1,further comprising bolts, wherein:said first through fourth plates haverespective, elongated holes formed therethrough, said elongated holesextending in a direction in which said respective edge surfaces of saidplates extend, respective said bolts being inserted through saidelongated holes and screwed into corresponding threaded holes in saidholder to fix said first through fourth plates to said holder.
 7. Themask as claimed in claim 1, wherein said holder is a ring-shaped platehaving a hole at a center thereof and having a first groove and a secondgroove formed in a surface thereof, said first groove and said secondgroove extending across said hole, said first groove being deeper thansaid second groove and receiving said first and second plates and saidsecond groove receiving said third and fourth plates.
 8. The mask asclaimed in claim 1, wherein each of said edge surfaces comprises asurface finished by a lapping process.
 9. A device for exposing a waferto a charged-particle beam after passing said charged-particle beamthrough at least one aperture to shape a cross section of saidcharged-particle beam with said aperture, said device comprising:meansfor generating said charged-particle beam; a mask having said at leastone aperture, said mask comprising first through fourth plates, eachplate having a respective edge surface which is a remaining portion of acorresponding side surface of the plate after beveling the plate; and aholder on which said first through fourth plates are fixedly mounted;and said first and second plates being arranged in parallel with eachother with said edge surface of said first plate facing said edgesurface of said second plate, said third and fourth plates beingarranged in parallel with each other with said edge surface of saidthird plate facing said edge surface of said fourth plate, said firstand second plates being arranged perpendicular to said third and fourthplates so that said respective edge surfaces of said first throughfourth plates form a rectangular aperture as said at least one apertureof said mask.
 10. The device as claimed in claim 9, wherein each of saidedge surfaces has a width e, a tolerable error of a pattern exposed onsaid wafer being denoted by α, a reduction rate of said rectangularaperture projected on said wafer being denoted by M, an angle of saidcharged-particle beam incident on said wafer being denoted by θ, andwherein said width e, said tolerable error α, said reduction rate M, andsaid angle θ are related as:

    e≦α·M/(2 tan θ),

and said width e is larger than a range of widths through which chargedparticles of said charged-particle beam travel in said first throughfourth plates before being stopped by collisions and is larger than alower limit of a mechanical process for forming said edge surface. 11.The device as claimed in claim 10, wherein a dimension D of surfaceirregularity of each of said edge surfaces is related as:

    D≦α·M.


12. The device as claimed in claim 11, wherein said mask furthercomprises bolts, said first through fourth plates having respective,elongated holes formed therethrough, said elongated holes extending in adirection in which said respective edge surfaces extend, respective saidbolts being inserted through said elongated holes and screwed intocorresponding threaded holes in said holder to fix said first throughfourth plates to said holder so that positions of said first throughfourth plates in said direction are adjustable, thereby to use aselected, sufficiently smooth portion of each of said edge surfaces forforming said rectangular aperture.
 13. The device as claimed in claim11, wherein said mask further comprises pins inserted through respectiveholes formed through said first through fourth plates and said holderand fixing said first through fourth plates to said holder.
 14. Thedevice as claimed in claim 9, wherein said mask further comprises bolts,said first through fourth plates having respective, elongated holesformed therethrough, said elongated holes extending in a direction inwhich said respective edge surfaces of said plates extend, respectivesaid bolts being inserted through said elongated holes and screwed intocorresponding threaded holes in said holder to fix said first throughfourth plates to said holder.
 15. The device as claimed in claim 9,wherein said holder is a ring-shaped plate having a hole at a centerthereof and having a first groove and a second groove formed in asurface thereof, said first groove and said second groove extendingacross said hole, said first groove being deeper than said second grooveand receiving said first and second plates, and said second groovereceiving said third and fourth plates.
 16. The device as claimed inclaim 9, wherein each of said edge surfaces comprises a surface finishedby a lapping process.
 17. A method of forming a mask having at least oneaperture and used in a device for exposing a wafer to a charged-particlebeam, said device passing said charged-particle beam through said atleast one aperture to shape a cross section thereof before exposing saidwafer to said charged-particle beam, said method comprising the stepsof:a) forming first through fourth plates; b) beveling each of saidfirst through fourth plates to form on each plate a respective edgesurface which is a remaining portion of a corresponding side surface ofthe plate after said beveling; and c) fixedly mounting said firstthrough fourth plates on a holder, said first and second plates beingarranged in parallel with each other with said edge surface of saidfirst plate facing said edge surface of said second plate, said thirdand fourth plates being arranged in parallel with each other with saidedge surface of said third plate facing said edge surface of said fourthplate and said first and second plates being arranged perpendicular tosaid third and fourth plates so that said respective edge surfaces ofsaid first through fourth plates form a rectangular aperture as said atleast one aperture of said mask.
 18. The method as claimed in claim 17,wherein said step c) further comprises the steps of:forming respectiveelongated holes through said first through fourth plates, said elongatedholes extending in a direction in which said respective edge surfaces ofsaid plates extend; and inserting bolts through said elongated holes andscrewing said bolts into corresponding threaded holes in said holder tofix said first through fourth plates to said holder.
 19. The method asclaimed in claim 18, wherein said step c) further comprises the stepsof:forming further holes through said first through fourth plates andsaid holder; and inserting pins through said further holes to fix saidfirst through fourth plates to said holder.
 20. The method as claimed inclaim 17, further comprising a step of finishing each of said edgesurfaces by applying a lapping process thereto.
 21. A method of exposinga wafer to a charged-particle beam, said method comprising the stepsof:generating said charged-particle beam; passing said charged-particlebeam through at least one aperture formed in and extending through amask; and exposing said wafer to said charged-particle beam afterpassing said charged-particle beam through said at least one aperture,wherein said mask comprises: first through fourth plates, each platehaving a respective edge surface which is a remaining portion of acorresponding side surface of the plate after beveling the plate; aholder on which said first through fourth plates are fixedly mounted;and said first and second plates being arranged in parallel with eachother with said edge surface of said first plate facing said edgesurface of said second plate, said third and fourth plates beingarranged in parallel with each other with said edge surface of saidthird plate facing said edge surface of said fourth plate and said firstand second plates being arranged perpendicular to said third and fourthplates so that said respective edge surfaces of said first throughfourth plates form a rectangular aperture as said at least one apertureof said mask.